The present invention relates to a voltage-controlled oscillator, and in particular, to a voltage-controlled oscillator capable of continuously controlling the frequency of oscillation by covering a wide range.
Description of the Prior Art
With the increase in the operation speed and capacity of communications apparatuses, it has been desired to increase a frequency and to lower jitters in a voltage-controlled oscillator which is arranged as a constituent element of a phase-locked loop (PLL). In the oscillator, it is regarded as quite important to employ an LC oscillator as an oscillation mechanism.
The LC oscillator includes inductors having a fixed or variable inductance. To adjust the oscillation frequency of the LC oscillator to a desired frequency in which inductors have a fixed inductance, variable capacitors are employed to appropriately adjust a capacitance so as to control the oscillation frequency. In such a case, control of the oscillation frequency is limited to a relatively narrower range adjustable by the variable capacitors. FIG. 1 shows a circuit configuration of a conventional oscillator.
FIG. 2 illustrates a conventional oscillator in which variable inductors capable of appropriately adjusting its inductance are used. The conventional oscillator includes variable capacitors as well as variable inductors which constitute an LC oscillator. Japanese Patent Laid-Open No. 11-289220 entitled xe2x80x9cBroadband Voltage-Controlled Oscillatorxe2x80x9d describes an example of a prior art oscillator. In this example, variable inductors enable to set a discrete inductance, and a wide range of oscillator frequency is set up by adjusting a capacitance of the variable capacitors.
However, in the conventional Broadband Voltage-Controlled Oscillator as described in the above-mentioned gazette, an inductance of variable inductors is adjusted only in a discrete manner. To control the oscillator so that it covers a wide range of the oscillation frequency, it is essential to adjust a capacitance of the variable capacitors. That is, the variable inductors and capacitors are essential elements for the conventional oscillator. With these inductors and capacitors, the oscillation frequency of the voltage-controlled oscillator is widely controlled, which corresponds to a variety of requirements in the development of recent communications systems.
It is therefore an object of the present invention to provide a voltage-controlled oscillator in which variable inductors capable of controlling its inductance in a continuous manner are adopted in place of the prior art variable inductors with which an inductance is controlled in a discrete fashion. The present invention also aims to provide a voltage-controlled oscillator which eliminates a process of adjusting a capacitance of the variable capacitors when performing a wide-range control of the oscillation frequency.
Another object of the present invention is to provide a voltage-controlled oscillator which comprises variable capacitors so that an accurate control of the oscillation frequency is performed by adjusting a capacitance of the variable capacitors and an inductance of the variable inductors.
To achieve the above-mentioned object, there is provided a voltage-controlled oscillator, comprising an LC oscillator including oscillator inductors, and control inductors respectively arranged in the vicinity of the oscillator inductors, wherein a mutual inductance is induced between the oscillator inductors and control inductors by supplying a control current to the control inductors.
In accordance with the present invention, the voltage-controlled oscillator further comprises a current source to supply the control current to the control inductors, in which the current source adjusts the control current in accordance with a control signal supplied from an external device.
In accordance with the present invention, there is provided a voltage-controlled oscillator, comprising an LC oscillator including oscillator inductors, and a plurality of control inductors arranged in the vicinity of each of the oscillator inductors, wherein a mutual inductance is induced between the oscillator inductors and the control inductors by supplying a control current to the control inductors.
In the voltage-controlled oscillator according to the present invention, the plurality of control inductors are arranged with a different spacing with respect to said oscillator inductors. Furthermore, the plurality of control inductors have an inductance of different value.